Diffusion methods to determine different forms of nitrogen in soil hydrolysates

Conventional steam-distillation techniques for fractionating the N in soil hydrolysates have generally indicated little variation in the chemical distribution of soil organic N, regardless of soil type, cropping, cultivation, or N-fertilization history. Nitrogen-15 tracer studies to evaluate these techniques showed that determinations of amino sugar-N are subject to serious underestimation, and that analyses for amino acid-N are vitiated by incomplete conversion of amino acid-N to NH₄-N following incomplete removal of hydrolyzable NH₄ and amino sugar-N. Diffusion methods were developed for fractionating the N in soil hydrolysates that are far more accurate and specific than steam distillation, while also being much simpler and more convenient. In these methods, total hydrolyzable N is measured by Kjeldahl digestion of the hydrolysate and diffusion of the digest with NaOH; diffusion is performed with MgO to determine hydrolyzable NH₄-N; (NH₄ + amino sugar)-N is recovered by diffusion with NaOH, after which amino acid-N is liberated by ninhydrin oxidation at pH <1.8 and recovered by diffusion with NaOH. Analytical accuracy and specificity were evaluated using a wide variety of purified organic-N compounds and by checking the recover of ¹⁵N added to soil hydrolysates as (NH₄)₂SO₄, glucosamine, or glycine. Studies using a diverse set of soils showed that distillation and diffusion usually agreed to within 10% for quantitative analysis of total hydrolyzable N, NH₄-N, and amino acid-N, whereas analyses of amino sugar-N were 74 to 317% greater by diffusion than by distillation.